Major efforts have been put in recent years on the development of Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) in order to achieve throughput and coverage improvement. In 3GPP LTE systems, some subframes can be allocated for enhanced physical downlink control channel (EPDCCH) for transmitting downlink control information (DCI). According to current 3GPP LTE specifications, a DCI (herein also called as EPDCCH message) may be transmitted by an EPDCCH candidate including 1, 2, 4, or 8 enhanced control channel elements (ECCEs). Depending on the number of ECCEs included in an EPDCCH candidate, there are multiple aggregation levels for the transmission of the EPDCCH message, such as EPDCCH aggregation level 1, 2, 4, or 8. An EPDCCH search space defines the ECCEs included in a specific EPDCCH candidate for a specific aggregation level.
A Mobile station such as a user equipment (UE) in the systems does not know in advance the EPDCCH candidate that carries an EPDCCH message intended for it. In general, the mobile station may attempt to blindly decode each possible EPDCCH candidate for each EPDCCH aggregation level in each subframe. More specifically, the mobile station firstly determines the ECCEs included in each EPDCCH candidate for each aggregation level according to the search space definition, and then performs blind decoding of the EPDCCH candidates to obtain any EPDCCH message intended for it.
In existing 3GPP LTE systems, the search space for a certain aggregation level is defined using a hashing function. For an EPDCCH PRB (physical resource block) set p (there can be two EPDCCH PRB sets for a certain UE), the ECCEs in the EPDCCH search space corresponding to an EPDCCH candidate m of an aggregation level L are given by:
                              L          ⁢                      {                                          (                                                      Y                                          p                      ,                      k                                                        +                                      ⌊                                                                  m                        ·                                                  N                                                      ECCE                            ,                            p                            ,                            k                                                                                                                      L                        ·                                                  M                                                      p                            ⁢                                                                                                                                                                      (                            L                            )                                                                                                                ⌋                                                  )                            ⁢              mod              ⁢                              ⌊                                                      N                                          ECCE                      ,                      p                      ,                      k                                                        /                  L                                ⌋                                      }                          +        i                            (        1        )            
whereYp,k=(Ap·Yp,k-1)mod D; 
i=0, . . . , L−1;
k represents the index of the current subframe, which may be in a range from 0 to 9 as ten subframes, subframes 0 to 9, are indexed in a frame;
NECCE,p,k represents the number of ECCEs included in the EPDCCH PRB set p of the subframe k;
m=0, 1, . . . Mp(L)−1;
Mp(L) represents the number of EPDCCH candidates for the aggregation level L in the EPDCCH PRB set p;
Yp,k represents a random number used for the randomization of the search space, Yp,−1=nRNTI≠0, A0=39827, A1=39829, D=65537; and
nRNTI represents the cell radio network temporary identifier (C-RNTI) of the UE.
With the starting positions of all EPDCCH candidates and the aggregation levels of the EPDCCH candidates, a base station such as evolved Node B (eNB) may select EPDCCH candidates for transmitting DCI for UE required resource allocation based on some scheduling strategy. One EPDCCH candidate may be selected for the DCI intended for a respective UE.
The UE may also determine the starting positions of all EPDCCH candidates for all aggregation levels in each subframe. By knowing the position of ECCEs included in each of the EPDCCH candidate, the UE may detect in each EPDCCH candidate via blind decoding to see whether there is DCI intended for it.
However, blind decoding of all the possible EPDCCH candidates in each subframe may cost a lot of power consumption. Existing EPDCCH search space determination is not suitable for the use cases where UE requires power consumption reduction. For example, UE for machine type communication (MTC UE) generally requires low power consumption to target ultra-long battery life and is thus desired to cost as low as possible during DCI detection. Moreover, since some UEs, such as MTC UEs deployed in challenging locations (also referred to as MTC UEs in coverage enhanced mode) may experience poor channel quality, eNB may transmit EPDCCH to such UEs using repetition. It is especially desirable that the power consumption for these UEs during EPDCCH detection could be as low as possible. Besides, LTE Rel.13 MTC UE only needs to support 1.4 MHz (i.e., only 6 PRB pairs) radio frequency (RF) bandwidth in downlink and uplink within any system bandwidth. A UE can be assigned with one or multiple UE-specific regions after initial access, depending on whether frequency hopping would be used for EPDCCH transmission.